The present invention relates to methods and apparatus for purification of water by filtration. More specifically, it concerns filtration systems with an upflow filter which contains a buoyant filter media in series with a downflow filter which contains a nonbuoyant filter media. In addition, the invention also concerns upflow filters with a buoyant filter media used in such a system.
It has been known for some time that water can be filtered by passing it upwardly through a bed of filter media comprising grains or small pellets of a buoyant material. An early example is shown in U.S. Pat. No. 468,984 of Boeing, issued in 1892, in which wood cubes are utilized for the buoyant material. Buoyant media out of synthetic materials such as polyethylene have also been used. Such a filtration method is also shown in United Kingdom patent specification No. 833,327 to Smith. A related upflow filtration process is described in Example 4 of U.S. Pat. No. 3,343,680 (Rice et al.).
These filters are contained within substantially closed tanks or in tanks internally divided by screens to prevent escape of the buoyant media. This makes the filtration zones and media beds inacessible during filtration, thereby limiting control over the filtration process. These filters also lack effective methods of cleaning the media in them. Backwashing of such filters has proved to be wasteful of energy, finished water and time since a very large downward flow of liquid is required before the media particles will separate. Mechanical agitation of the media, as described in the Smith specification, enhances cleaning, but requires a wasteful consumption of energy in order to sufficiently agitate a packed media bed that impurities are released.
Still another example of a buoyant media filter is shown in U.S. Pat. No. 4,115,266 of Ohshima. In Ohshima, air is injected through small holes in a pipe below the filter bed for backwashing the filtering materials or aeration of adhered purifying bacteria. Ohshima also mentions arranging his filters in series, but is apparently only concerned with filters operating at very slow rates so as to permit growth of oily matters and suspended substances on the surfaces of the buoyant media particles. Faster flow rates are understood to tend to agitate the media bed and interfere with this approach to filtering.
Although buoyant media filters have been known for at least close to a century, they have heretofore been used in relatively low flow rate applications because a practical method and apparatus for using such filters in a high rate water treatment plant was previously unknown.